Furnace Size Calculator: What Size Furnace Do I Need? BTU by Sq Ft

What Size Furnace Do I Need?

Figuring out exactly what size furnace you need is more complex than just looking at the square footage of your house. To get it right, you need to calculate the Output BTU (the actual heat your home requires to stay warm) and then map that to the Input BTU of the furnace you intend to buy (how much fuel it burns).

Our calculator takes the guesswork out of this by running a sequential estimate built on the BTU per square foot rates the trade actually sizes furnaces with. It factors in your climate, your home's physical structure, and the efficiency of the equipment. Be aware of what that means: these rates carry deliberate margin and are calibrated closer to an older, leakier house than a tight modern one. If your home is well sealed and well insulated, a true room-by-room load calculation will often come out well below this figure, sometimes by half. Use our heat load calculator for the load-based number, and treat this page as the shopping size.

If you are considering an electric alternative to gas heating, you can also calculate your requirements using our Heat Pump Size Calculator, as heat pumps are sized differently than traditional furnaces.

Once you know the input-BTU size you need, see what the swap will actually cost with the furnace replacement cost calculator - installed 2026 prices by size, AFUE tier, and region. And since the AC coil sits on top of the furnace, it pays to size the cooling side at the same time with our AC unit size calculator.

How Furnace Size Is Calculated

To estimate furnace sizing, professionals use a calculation called the Manual J Load Calculation. While highly complex, the fundamental math is straightforward:

1. Base BTU Load: Multiply your heated square footage by a baseline BTU rate determined by your climate zone (e.g., 30 BTUs in Florida vs 60 BTUs in Minnesota).
2. Environmental Multipliers: Increase the load if the home has poor insulation, drafty windows, or high ceilings. Decrease the load for excellent insulation or shared walls (like townhomes).
3. Efficiency Conversion: Divide the final required heat load by the furnace's AFUE percentage to find the size of the furnace you actually need to buy off the shelf.

This calculator runs exactly those three steps, and every number behind them is published rather than hidden. The climate BTU rates, the insulation, window, layout and ceiling multipliers, and the electric kilowatt ladder used when you pick electric instead of gas are all listed in our furnace sizing methodology.

BTU vs AFUE: What Furnace Size Really Means

One of the biggest mistakes homeowners make is confusing the "size" listed on the furnace box with the actual heat the furnace delivers.

• Input BTU: The number printed on the furnace (e.g., a "60k BTU furnace"). This is how much gas it consumes.
• AFUE (Efficiency): The percentage of that gas that turns into usable heat.
• Output BTU: The actual heat that makes it into your living room.

If your home needs 60,000 BTUs of heat, you cannot buy an 80% efficient 60,000 BTU furnace! That furnace will only deliver 48,000 BTUs of heat (60,000 × 0.80), leaving you freezing in the winter. Instead, you would need to buy a 75,000 BTU furnace to hit your 60k Output target.

Furnace BTU by Climate Zone

Your climate is the single most important variable in determining furnace size. Homes in colder regions require drastically more baseline heat per square foot.

• Zone 1 (Hot - e.g., Miami, Houston): 30–35 BTU per sq ft.
• Zone 2 (Warm - e.g., Atlanta, Dallas): 35–40 BTU per sq ft.
• Zone 3 (Mixed - e.g., DC, St. Louis): 40–45 BTU per sq ft.
• Zone 4 (Cool - e.g., Boston, Chicago): 45–50 BTU per sq ft.
• Zone 5 (Cold - e.g., Minneapolis, Fargo): 50–60 BTU per sq ft.

Furnace Size for Townhouses, Condos, and Shared Walls

If you live in a townhouse, condo, or duplex, do not use a basic square footage calculator.

Shared walls do not lose heat to the outdoors. A middle-unit townhouse has massive "insulation" on both sides because the neighbors are heating their homes too! Our calculator specifically includes a Home Type adjustment that reduces your heating load by up to 15% if you have shared walls, ensuring you don't accidentally buy a massively oversized furnace.

How Insulation, Windows, and Layout Change Furnace Size

Heat naturally escapes your house. A 1950s home with single-pane windows and poor attic insulation bleeds heat rapidly, while a modern, tightly sealed home traps it inside. Our calculator applies penalties up to +15% for poor insulation and drafty windows, and discounts down to -18% for excellent modern sealing.

What Happens If a Furnace Is Too Big?

Bigger is not better when it comes to HVAC. If you buy a furnace that is too large for your home (oversized), it will suffer from short cycling. The furnace will blast the house with heat, rapidly reach the thermostat temperature, and abruptly shut off. Ten minutes later, it turns back on. This constant on-and-off cycling causes:

• Spikes in your energy bill.
• Uneven temperatures (the room with the thermostat is hot, but bedrooms stay freezing).
• Massive wear and tear on the heat exchanger and blower motor.

There is a second problem people rarely connect to furnace size. Forced-air heat dries the indoor air out whatever size the furnace is, and short cycles make it worse by moving a lot of hot, dry air in bursts rather than gently over a long run. If winter static shocks and cracked lips are part of the picture at your house, our whole-house humidifier calculator sizes the unit that mounts on the supply plenum.

What Happens If a Furnace Is Too Small?

If your furnace is undersized, it will run continuously during deep winter freezes and never successfully reach your target temperature on the thermostat. While long run-times are actually good for efficiency, an undersized unit will simply leave you cold during extreme weather events.

Gas or Electric Furnace: The Numbers Nobody Gives You

Ask this anywhere and you get “gas is cheaper” with nothing behind it. Here is the arithmetic, because the answer is not universal and it turns entirely on what your two utilities charge.

Compare on cost per 100,000 BTU actually delivered into the house, which is the only fair basis, since gas has to overcome its own flue losses and electric does not. Electric resistance: 100,000 BTU is 29.3 kWh, and at 17 cents that is $4.98. Gas at $1.20 a therm through an 80% furnace: you burn 1.25 therms to deliver 100,000, so $1.50. Gas is about 3.3 times cheaper to run at those rates. Through a 96% furnace it is 4 times cheaper, because less goes up the flue.

Now the part that makes it a real decision rather than a slogan. Those rates are not yours. Run your own: electricity would have to be around 5 cents per kWh to match $1.20 gas, which almost nowhere in the country is. But gas at $2.00 a therm narrows the gap to about 2 times, and in places with cheap hydro power and expensive delivered propane the ordering can flip outright. Take your latest bills, find the per-kWh and per-therm figures, and do those two multiplications before you accept anyone's rule of thumb.

For sizing, nothing changes: an electric furnace for a house is picked from the same heat load, then converted to kilowatts. Choose Electric in the calculator above and it does that step for you. As a reference point, an electric furnace for a 2000 sq ft home in a mixed climate works out near 25 kW, and the common residential range runs 10 to 25 kW. If you are fitting an electric furnace with AC on the same ductwork, the blower still has to satisfy the cooling side, so use the paired AC tonnage field in the advanced options: an electric furnace and AC sharing one air handler is sized on whichever of the two needs more air, and that is almost always the AC.

Heat Pump vs Electric Furnace, and When a High-Efficiency Electric Furnace Still Wins

Two things that change the picture and get left out of the comparison entirely. An electric furnace is cheaper to buy and install, because there is no flue to run, no gas line, no combustion air and no carbon monoxide risk, which is exactly why builders fit them and why they are standard in manufactured housing. So a high efficiency electric furnace can still be the right call in a mild climate where you barely run it, or where there is no gas service at the street and bringing it in costs thousands.

And if you are going all-electric anyway, resistance heat is the wrong way to do it. A heat pump moves heat rather than making it and delivers roughly three units of heat per unit of electricity, so it undercuts an electric furnace by about two thirds on the same tariff. The common compromise is dual fuel: a heat pump doing the work down to a lockout temperature you set, around 35 to 40°F, with the gas furnace taking over below that. Size the heat pump side with the heat pump size calculator and use the electric option above for the backup element.

When an Online Estimate Is Enough vs Manual J

An online estimate is perfect for gut-checking quotes from contractors, budgeting for a replacement, or narrowing down your shopping options. However, if you live in an extremely cold climate (Zone 5) and have a very old, drafty home, the variables become highly unpredictable. In these extreme "edge cases," or when building a custom new home, a professional ACCA Manual J Load Calculation should always be performed before writing a check.

Furnace Airflow: How Much CFM Your Blower Has to Move

Sizing the furnace is only half the job. The blower inside it has to move a matching volume of air, and if the ductwork cannot carry that volume you get the same complaints as an undersized furnace: cold rooms at the end of the run, noise at the registers, and a unit that runs far longer than it should.

The relationship is heat delivered = 1.08 × CFM × temperature rise. Two things about that are worth pinning down, because both catch people out.

First, airflow follows the output BTU, not the input rating on the box. A 100,000 BTU furnace at 80% AFUE only delivers 80,000 BTU into the house, so it is the 80,000 that sets the airflow. Second, the temperature rise is not a constant you can assume. It is printed on the furnace data plate and it varies, usually between 40 and 70°F. That range moves the answer more than most people expect: the same 80,000 BTU output needs about 1,852 CFM at a 40°F rise but only 1,058 CFM at 70°F.

Here is where the common furnace sizes land at a mid-range 50°F rise, assuming 80% AFUE, with the supply trunk each one implies:

Trunk sizes assume rigid metal at 900 FPM. Flex duct carries roughly a third less air at the same diameter, so it needs a step up. The return side needs to be larger again, because return air is held to a lower velocity. Our duct size calculator works both of those out from the CFM figure, and if you want the airflow arrived at a different way, such as from room-by-room air changes, the HVAC CFM calculator covers those methods.

Furnace Size Chart (By Square Footage & Climate)

Here is a quick reference chart for Output BTUs needed based on square footage and general climate zone. (Remember, you must divide these numbers by your desired AFUE to find the furnace size you should buy).

Furnace Sizing Guide by BTU: What Each Size Actually Heats

The chart above runs from house size to furnace size. Most people arrive with the opposite question: they are looking at a specific furnace and want to know whether it fits their house. So this furnace sizing guide works the other direction, one size at a time.

Two things carry through every section below. The square footage is a range, not a number, because a furnace that heats 1,164 sq ft in Minnesota heats 1,969 sq ft in Georgia. And each size lists its blower airflow and the supply trunk that airflow needs, since a correctly sized furnace on undersized ductwork behaves like an undersized furnace. Airflow figures assume a 50°F temperature rise and rigid metal duct at 900 FPM.

40000 BTU Furnace: The Smallest Size Worth Installing

At 80% AFUE a 40,000 BTU furnace delivers 32,000 BTU of actual heat, covering roughly 580 sq ft in a cold climate and 985 sq ft in a warm one. It needs about 593 CFM, which a 12-inch supply trunk handles. This is the floor of the residential range, and it suits condos, small apartments, additions and well-sealed cottages. Below this you are into wall furnaces and ductless heating rather than a ducted system.

50000 BTU Furnace and the Gap Above It

A 50,000 BTU unit puts out 40,000 BTU, good for about 727 to 1,231 sq ft depending on climate, on 741 CFM through a 14-inch trunk. Worth knowing that manufacturers thin out here. The common ladder runs 40k, 60k, 80k, 100k, so 50k and 70k are made by fewer brands and stocked by fewer distributors. If your load lands at 50,000 you will often be choosing between waiting for a 50k or taking the 60k that is on the truck.

60000 BTU Furnace for a Small to Mid Size Home

Output is 48,000 BTU, which covers roughly 873 sq ft in Zone 5 and 1,477 sq ft in Zone 1. Airflow is 889 CFM, still inside a 14-inch trunk. This is where the climate spread starts to matter for real. The same 60,000 BTU furnace is right for a 900 sq ft house in Minneapolis and a 1,450 sq ft house in Atlanta. Anyone quoting furnace size from square footage alone, without asking where you live, is guessing.

70000 BTU Furnace Where the Trunk Steps Up

56,000 BTU of output, about 1,018 to 1,723 sq ft, and 1,037 CFM of airflow. That airflow is the notable part: 70,000 BTU is where the supply trunk moves from 14 to 16 inches, because 1,037 CFM through a 14-inch duct runs past the comfortable velocity limit. If you are replacing a 60k with a 70k in an older house, check the existing trunk before assuming it carries over.

75000 BTU Furnace: The Odd Size in the Middle

A 75,000 BTU furnace delivers 60,000 BTU and covers about 1,091 to 1,846 sq ft on 1,111 CFM through a 16-inch trunk. It exists mainly because 60,000 output is a round number that load calculations land on often. Functionally it sits so close to the 80k that most contractors will quote whichever they stock, and the difference in a real house is small enough not to argue about.

80000 BTU Furnace, the Most Common Residential Size

Output 64,000 BTU, coverage roughly 1,164 sq ft cold to 1,969 sq ft warm, airflow 1,185 CFM on a 16-inch trunk. This is the size most often installed in North American homes, which is exactly why it is also the size most often installed incorrectly. It is the default a contractor reaches for when nobody has run a load calculation. In a mixed climate it fits a house of about 1,500 sq ft, so if yours is 1,100 and you have been quoted an 80k, ask what the quote was based on.

90000 BTU Furnace for Larger or Colder Homes

72,000 BTU output covers about 1,309 to 2,215 sq ft, needing 1,333 CFM and an 18-inch trunk. The duct step is the practical constraint at this size. Plenty of houses that could use a 90k furnace have ductwork built for a 60k, and dropping the bigger unit onto the smaller trunk produces noise at the registers and a blower working against itself rather than more heat in the far bedrooms.

100000 BTU Furnace and What It Actually Heats

Output is 80,000 BTU, covering roughly 1,455 sq ft in a cold climate up to 2,462 sq ft in a warm one, on 1,481 CFM through an 18-inch trunk. The spread here is a thousand square feet wide, which is the clearest illustration on this page of why climate zone is not a minor adjustment. A 100,000 BTU furnace is a large-home unit in Texas and a mid-size-home unit in Wisconsin.

120000 BTU Furnace at the Top of Residential

A 120,000 BTU furnace delivers 96,000 BTU, covering about 1,745 to 2,954 sq ft, and needs 1,778 CFM through a 20-inch trunk. Past this point single-furnace installs get rare. Homes needing more are usually better served by two smaller furnaces zoned separately, partly because one blower struggles to move that much air through a typical residential duct system, and partly because a single 120k short-cycles badly in shoulder season when the house needs a fraction of its output.

Why a High-Efficiency Furnace Heats More House at the Same Nameplate

Every figure above assumes 80% AFUE, the standard tier. Swap to a 96% condensing furnace and the same nameplate number delivers considerably more heat, because less of it leaves through the flue.

An 80,000 BTU furnace at 96% delivers 76,800 BTU instead of 64,000, which in a mixed climate takes its coverage from about 1,506 sq ft to roughly 1,807 sq ft. So if you are moving from an old 80% unit to a 96% one, you often need a smaller nameplate rating, not the same one. Matching the old number is one of the most common ways a high-efficiency replacement ends up oversized. It also raises the airflow requirement to 1,422 CFM, which pushes the trunk from 16 to 18 inches.

The same step applies all the way up the ladder. A 60,000 BTU furnace at 96% efficiency delivers 57,600 BTU instead of the 48,000 an 80% unit puts out, and a 100,000 BTU furnace at 96% efficiencydelivers 96,000 BTU instead of 80,000 - each one roughly a size's worth of extra usable heat from the same nameplate. That is the arithmetic behind the rule: multiply the input BTU by 0.96 to get the real output before you compare it against what your home needs.